We conducted patient-oriented clinical research in neurocardiology. Studies focused on elucidating pathophysiologic mechanisms and developing novel diagnostic and therapeutic approaches for disorders involving altered regulation of catecholamine systems. These conditions result from dysfunction of the autonomic nervous system (dysautonomia) or abnormally decreased or increased production of the catecholamines, norepinephrine (NE), epinephrine (EPI), or dopamine (DA). Patients with autonomic failure in the setting of Parkinson's disease all had cardiac sympathetic denervation, detected by 6-[18F]fluorodopamine positron-emission tomographic scanning. In contrast, patients with multiple system atrophy, which can be difficult to distinguish clinically from Parkinson's disease, all had evidence for intact cardiac sympathetic nerve terminals. Even in the absence of autonomic failure, most patients with Parkinson's disease had evidence for localized or diffuse loss of cardiac sympathetic nerve terminals. Cardiac sympathetic denervation in Parkinson's was found to be independent of levodopa treatment and can arise from mutation of the gene encoding alpha-synuclein. In the diagnostic evaluation of pheochromocytoma, a clinically important tumor that produces catecholamines, plasma levels of metanephrines, metabolites of NE and EPI made in the tumor, provided a uniquely and virtually perfectly sensitive screening test. In patients with chronic orthostatic intolerance, a pattern of increased adrenomedullary hormonal system activity and inhibition of sympathetic nervous system activity ("sympathoadrenal imbalance") was found to precede tilt-induced and spontaneously occurring neurocardiogenic syncope. 6-[18F]Fluorodopamine positron-emission tomographic scanning successfully localized the tumor even in difficult cases. A combined neurogenetic and neurochemical approach holds great promise for understanding how particular mutations in familial diseases associated with increased production of NE (pheochromocytoma) or decreased production of NE (Menkes disease) relate to particular neurochemical and clinical manifestations.